In order to warn a driver that the automobile is approaching an object in its vicinity, providing a millimeter wave radar apparatus for measuring distance on each part of an automobile, for example, behind the radiator grill, the side moldings, the back panels, and the like, is under study. However, in the case in which the radiator grill and the like are provided with a sheen by using a metallic coating, this metallic coating blocks or greatly attenuates the millimeter waves. Thus, the path of the millimeter waves of the radar apparatus must be covered by a radar apparatus cover that has a sheen and has millimeter wave transparency. In order for the metallic coating to be transparent to millimeter waves, a discontinuous structure is necessary. In this discontinuous structure, the metallic coating does not form one continuous surface, but instead it has a structure (sea-island structure) in which many fine metal films are spread over the surface in a state in which they are slightly separated from each other or have portions thereof in contact so as to coalesce into islands.
In the case of the conventional millimeter wave radar apparatus cover, the metallic coating having a sheen and discontinuous structure is formed by depositing a single metal such as In or Sn by using a vacuum vapor deposition method. This is because metals such as In, Sn and the like have the quality of readily forming a discontinuous structure. Most metals do not exhibit this quality significantly, and if they are deposited by using vacuum vapor deposition, the metal becomes continuous at the point in time when enough of the metal has been deposited to obtain a region of a coating thickness that attains a sheen that is adequate in terms of external appearance. Consequently, the electrical resistance becomes low, and the millimeter wave transparency becomes insufficient.
(1) However, there are problems in that In in particular is expensive and the product cost becomes high. Thus, reducing the amount of In that is used and using metals other than In are being pursued.
(2) In addition, when metals such as In and Sn, which readily form a discontinuous structure by using vacuum vapor deposition, are deposited by using sputtering, it has not been possible to form a sufficiently distinct discontinuous structure, and as a result, a substantially continuous structure is acquired, the electrical resistance becomes low, and the millimeter wave transparency becomes insufficient. In addition, when a continuous structure has been acquired, corrosion of the metallic coating spreads easily and the corrosion resistance is thereby reduced. Thus, corrosion may occur, for example, due to the heat applied during the insert injection molding of an AES resin or the like that is carried out during the manufacturing process of the millimeter wave radar apparatus cover, and defects in the external appearance may thereby be caused. However, because sputtering is characterized in having both a superior productivity and a lower cost compared to vacuum vapor deposition, establishing a manufacturing method for a metallic coating having a sheen and a discontinuous structure by using sputtering is being pursued.
The following patent documents are examples of prior art that relates to the present invention.
Japanese Examined Patent Application Publication No. JP-B-S59-40105 discloses a metallic thin film made by sputtering a stainless steel or a nickel-chrome alloy being provided on a pliable and shiny product such as a front grill.
Japanese Patent Application Publication No. JP-A-2005-249773 discloses a molded product disposed in a radar apparatus beam path that provides a shiny decorative layer made by vacuum vapor depositing or sputtering In, an In alloy, Sn, or a Sn alloy, on a base material surface made of a cyclic polyolefin.
Japanese Patent Application Publication No. JP-A-H10-193549 discloses a decorative trim and laminated film in which a metallic thin film is formed on a transparent film layer by subjecting Pb, Al, Sn, In or an alloy thereof to a vacuum metallization treatment such as sputtering, resistance heating vacuum vapor deposition, or an electron beam method.